The present invention relates to data communication and more particularly, to data communication using spread spectrum techniques. The invention also relates to communication applications using signature sequences.
Spread spectrum communication techniques are used for information carrying signals in a variety of communication systems because of their ability to reduce the effects of certain transmission impairments. Many multi-user communication techniques suffer co-channel interference, multiple access interference and intersymbol interference. The use of spread spectrum transmission and reception attenuates these interference types.
In Local Area Networks (LANs) there is an increasing need for wireless access. This wireless access allows mobile computer users to remain in contact with a given corporate LAN over short distances. Currently available systems provide such connections using either radio or infrared communication technology. For certain system requirements, this communication is adequate. However, the data transmission rates achievable are relatively low and this significantly limits the number of applications to which the systems may be applied and implementation costs are often prohibitive.
Wireless local area network (WLAN) products were thus for a long time a specialty, made available by a small number of vendors and built accordingly to meet proprietary requirements. The Institute of Electrical and Electronic Engineers (IEEE) in June 1997 formalized a standard that will control interoperability of such products known as 802.11. While this standard will undoubtedly promote the growth of WLAN products, the problems of transmission rates, reliability and cost remain. One possible solution is obtained by the application of spread spectrum communication techniques using signature sequences. One such approach is identified in “A 2.4 GHz 11 MBps Baseband Processor for B02.11 Applications”, ANDREN; Harris Semiconductor (May 05, 1998). While implementations of this type overcome the traditional problems, it is difficult to synchronise data communication without the use of complex circuitry. Synchronisation difficulties include chip and symbol synchronisation as well as the problem of signal strength measurement.
One solution to these problems has been proposed by Harris (i.e. the Intersil PRISM radio chip set for use in the new 802.11 WLAN standard at 11 Mb/s. In common with most proposed solutions there is a phase involving the acquisition of synchronisation and a phase involving the maintenance. Acquisition in this case, is accomplished using a single correlator and an embedded Barker sequence. Synchronisation is maintained using an early-late detector. See “A 156-MB/S Interface CMOS LSI for ATM Switching Systems”, KOSAKI T. et al leice Transactions on Communication, JP, Institute of Electronics Information and Communication Engineering, Tokyo (Jan. 06, 1993), E76-B (6), 684–693. While the previously known ‘Harris type’ early-late detector solution is practical in a wide variety of applications, the early-late detector operates directly on the incoming sequence stream, which is composed of binary codes at the input. This leads to reliability problems in that quality of the signal cannot be guaranteed.
There is therefore a need for a method and apparatus, which will overcome the aforementioned problems.